I can't remember where I saw/read the idea but it's also possible to make char cloth without using a tin at all. Silver foil, in the demo it was food wrappings I think, but I used kitchen foil folded to two thicknesses. Place the cotton material in the foil and fold the edges over to make a seal and put it in the fire.
I did it on a gas cooker and didn't make an air hole in the foil, smoke found it's way out of the folded 'seams'... Mucho smoko!! But it did work..so too did my smoke alarm..
Thinking about it, those heavy foil containers that Chinese take-aways come in, flattened out and folded over would probably be ok but I'm struggling to remember why he didn't just use a tin like everybody else...